According to Adelaide Now, the boys' mother was so amused by the letter she posted it on Facebook, and naturally it went viral.
Here's the full text of Zac's note.
To the best brother ever!
I'm very sorry that I pinched you in the nuts. I'm so sad that I feel the pain. I know that this note won't completely make it up for you so heres $1.00 all for you! I know it doesn't make the pain go away and I can't put a band-aid on it! You wouldn't be able to pee! I hope you better in the morning buddey!
Love from Zac.
Swim Week Miami Beach 2014 [PHOTOS]